Cathode ray tube acoustic transducers

ABSTRACT

An acoustic transducer comprises a cathode ray tube which has an acoustically transmissive faceplate and a layer of piezoelectric material, disposed internally of the tube supported by the faceplate. A layer of resilient material, which is acoustically transmissive and vacuum compatible is interposed between the layer of piezoelectric material and the faceplate to provide vibrational decoupling between them.

ite States tet Brown et a1.

[ Aug. 12, 1975 CATHODE RAY TUBE ACOUSTIC TRANSDUCERS Inventors: Patrick Harold Brown, l-lillingdon; Raymond Frank Sivyer, Sunbury-on-Thames, both of England 3,505,558 4/1970 Jacobs 313/369 X 3,594,583 7/1971 Sheldon 313/369 X 3,622,825 11/1971 Bennett 313/369 Primary ExaminerRobert Segal Attorney, Agent, or FirmFleit & Jacobson [57] ABSTRACT An acoustic transducer comprises a cathode ray tube which has an acoustically transmissive faceplate and a layer of piezoelectric material, disposed internally of the tube supported by the faceplate. A layer of resilient material, which is acoustically transmissive and vacuum compatible is interposed between the layer of piezoelectric material and the faceplate to provide vibrational decoupling between them.

7 Claims, 1 Drawing Figure 1 CATI-IODE RAY TUBE ACOUSTIC TRANSDUCERS This invention relates to cathode ray tube acoustic transducers.

In the construction of an ultrasonic sensing device for use in a television camera, it is necessary for the sensing element, usually a quartz piezoelectric target, to be attached to a vacuum-sealed support window whose physical properties are compatible with vacuum environment and processing, transmission of ultrasonic radiation, and the thermal expansion of the sensing element.

In US. patent application Ser. No. 432322, assigned to a common assignee, there is described a cathode ray tube having an acoustic transducer in which, inter alia, a piezoelectric quartz target of half-wave thickness is secured by an epoxy resin adhesive to a support window of plastics material.

Experiments have now indicated that the direct glueing of quartz onto a support window of a material having an acoustic impedance similar to that of the quartz may result in loss of the half-wave thickness resonance since the quartz and the support window tend to vibrate as a single unit, resulting in a degradation of resolution and sensitivity.

It is an object of the present invention to provide an improved cathode ray tube acoustic transducer.

According to the invention there is provided a cathode ray tube acoustic transducer including,

a. an evacuated envelope,

b. a faceplate of acoustically transmissive material sealed to said envelope,

c. a piezo-electric transducing layer supported by said faceplate and sensitive to spatial variations of pressure applied thereto,

(1. an electron gun mounted within said envelope for providing a beam of electrons directed toward said transducing layer,

e. means for scanning said beam,

f. signal electrode means for deriving electrical output signals from said transducing layer indicative of the spatial variations of pressure applied thereto and g. an acoustically transmissive layer of resilient material separating and in contact with said faceplate and said transducing layer.

In order that the invention may be clearly understood and readily carried into effect it will now be described by way of example with reference to the accompanying drawing in which the single FIGURE illustrates diagrammatically a sectional view of a cathode ray tube in accordance with the invention.

Referring to the drawing, a cathode ray tube has an envelope 1, of good ductile metal such as copper or aluminium. Alternatively, the envelope may be of vitreous material, eg glass. A conventional electron gun 2 produces a beam of electrons for scanning across the target. The electron beam may be focused and scanned in conventional manner by electrostatic or electromagnetic means indicated generally at 13. In the case of a glass envelope, a conventional wall anode 8 may be provided, with an electrical connection 4. Pins such as 11 are mutually insulated and hermetically sealed by bushes such as 12 through the envelope for effecting electrical connections to the various internal electrodes.

The envelope 1 is hermetically sealed to a faceplate 3 by means of a groove 7.'The faceplate is formed of a rigid material which will transmit the ultrasonic radiation image for which the device is employed. The terminal expansion coefficients of the faceplate, envelope and, if used, sealing material must be matched to avoid the introduction of thermal stresses which might lead to the breakdown of the hermetic seal. Suitable plastics materials for the faceplate are given by way of example in the aforementioned co-pending patent application. As a further example, the faceplate 3 may comprise Pyrex.

Supported on the inner surface of the faceplate 3 is a tessellated target structure comprising plates such as 6 of piezoelectric material. For example, the plates 6 may comprise X-cut crystals of quartz, spaced from each other to allow for expansion. The plates 6 are of a thickness such as to achieve half wave acoustic resonance therein. For example, for use with ultrasonic sound waves having a frequency of 2 MHz, the thickness of the plates 6 would be 1.43 mm. The target includes a signal electrode 5 and a hermetically sealed lead 9. I

In accordance with the invention, a resilient layer 10 is interposed between the target and the faceplate. The resilient layer 10 comprises a material which allows the ultrasonic energy to pass through to the target and cause the piezoelectric plates 6 to vibrate in the planar mode while at the same time providing sufficient vibrational decoupling between the plates 6 and the faceplate 3. The layer 10 must also be vacuum compatible so as not to adversely affect the vacuum in the cathode ray tube, and it must also be able to withstand tube processing such as baking. As an example, a resilient layer 10 comprising a layer of Viton rubber 0.8mm thick has been found most suitable.

Although the introduction of the resilient layer 10 causes a mismatch of acoustic impedances, sufficient ultrasonic energy is transmitted to the plates 6. The vibrational decoupling provided by the resilient layer 10 allows the resolution to be retained since the plates 6 and the faceplate 3 do not vibrate as a single unit. The resilience of the layer 10 also accommodates the differential thermal expansion encountered during vacuum process baking and, if required, the hot curing schedule for adhesives.

For a more detailed description of the operation of the cathode ray tube, attention is directed to the aforementioned U.S. Pat. application Ser. No. 432322. Briefly, however, the surfaces of the piezoelectric plates 6 are stabilized at some suitable potential by scanning them with the electron beam. When an ultrasonic image is received, the plates 6 are caused to vibrate in the planar mode in dependance upon the distribution of the ultra-sonic energy in the image. This causes a correspondingly distributed variation in the charge on the stabilized surfaces of the plates. The plates are then re-scanned to re-stabilize the surfaces, and in so doing, capacitive coupling produces signals in the signal electrode 5 which correspond to the distribution of the ultra-sonic energy in the image.

Various modifications within the scope of the invention will be apparent to anyone skilled in the art. For example the piezoelectric target may comprise material other than quartz, and may take forms other than a tessellated structure. Also, other resilient materials may be employed for the layer 10.

The envelope can conveniently be sealed to the faceplate 3 by extending the layer 10 beyond the periphery, of the envelope, and clamping the envelope to the faceplate so as to compress the extended part of the layer 10 between the end of the envelope and the faceplate.

The compression of the layer 10 produces an adequate seal, and moreover the presence of the Viton rubber between the envelope 1 and the faceplate l permits differential expansion to occur between the two elements.

What we claim is:

l. A cathode ray tube acoustic transducer including,

a. an evacuated envelope,

b. a faceplate of acoustically transmissive material sealed to said envelope,

c. a piezo-electric transducing layer supported by said faceplate within said envelope and sensitive to spatial variations of pressure applied thereto,

d. an electron gun mounted within said envelope for providing a beam of electrons directed toward said transducing layer,

e. means for scanning said beam,

f. signal electrode means for deriving electrical outranged such that part of said layer of resilient material may be compressed between said faceplate and said envelope to provide a hermetic seal.

3. An acoustic transducer according to claim 1 wherein said resilient layer is formed of Viton rubber.

4..An acoustic transducer according to claim 1 wherein said envelope is formed of glass.

5. An acoustic transducer according to claim 1 wherein said envelope is formed of metal.

6. An acoustic transducer according to claim 1 wherein said piezoelectric layer comprises a tesselation of smaller plates.

7. An acoustic transducer according to claim 1 wherein said faceplate is formed of Pyrex glass. 

1. A cathode ray tube acoustic transducer including, a. an evacuated envelope, b. a faceplate of acoustically transmissive material sealed to said envelope, c. a piezo-electric transducing layer supported by said faceplate within said envelope and sensitive to spatial variations of pressure applied thereto, d. an electron gun mounted within said envelope for providing a beam of electrons directed toward said transducing layer, e. means for scanning said beam, f. signal electrode means for deriving electrical output signals from said transducing layer indicative of the spatial variations of pressure applied thereto and g. an acoustically transmissive layer of resilient material separating and in contact with said faceplate and said transducing layer for maintaining the resonant frequency of said transducing layer.
 2. An acoustic transducer according to claim 1 arranged such that part of said layer of resilient material may be compressed between said faceplate and said envelope to provide a hermetic seal.
 3. An acoustic transducer according to claim 1 wherein said resilient layer is formed of Viton rubber.
 4. An acoustic transducer according to claim 1 wherein said envelope is formed of glass.
 5. An acoustic transducer according to claim 1 wherein said envelope is formed of metal.
 6. An acoustic transducer according to claim 1 wherein said piezoelectric layer comprises a tesselation of smaller plates.
 7. An acoustic transducer according to claim 1 wherein said faceplate is formed of Pyrex glass. 